PHP INTRODUCTION
PHP is server side language it execute web server and resulting html send to the browser .
PHP save file with file_name.php
All php code run in php tag (<?php write_code ?>) where echo used to display text to web page
index.php -This is file extension
Example
<DOCTYPE html>
<head>
<meta charset="utf-8">
<title> hello php </title>
</head>
<body>
<p><?php echo "hello word" ?> </p>
</body>
</html>
Php code writen within in php tag
Result
hello word
php is write in side php code .php code written only php file if you write php inside the html , css ,or javascript file then php code not run . php code run only php file and php file run on the server
Example

Localhost server mean it run php only for you and it working on you local network that can we easily access your localhost file in your mobile
Localhost server can not serve out side you local area network
Some server name that can be use in your computer for development time of php